This application claims the benefit of Korean Application No. 00-1782, filed Jan. 14, 2000, in the Korean Industrial Property Office, the disclosure of which is incorporated herein by reference.
1. Field of the Invention
The present invention relates to an optical pickup for high-density information writing and reading systems, and more particularly, to an optical pickup capable of reducing chromatic aberration that occurs when a blue light source is employed.
2. Description of the Related Art
In optical writing and reading systems, the recording density is determined by the size of the focused spot. In general, the size of the focused spot (S) is proportional to the wavelength (xcex), and inversely proportional to the numerical aperture (NA), as expressed by formula (1):
Sxe2x88x9dxcex/NAxe2x80x83xe2x80x83(1)
For a higher recording density than that for compact disks (CDs) or digital versatile disks (DVDs), the size of the spot being focused on an optical disk must be further reduced. To reduce the spot size, as can be inferred from formula (1), the wavelength (xcex) of a laser beam must be reduced and the NA of an objective lens must be increased. Thus, for high density information recording, a laser beam that has a short wavelength, such as a blue laser, must be employed as a light source and the NA of the objective lens must be maintained at 0.6 or more.
On the other hand, coma aberration (W31), which occurs due to a tilting of the optical disk, is associated with the tilt angle (xcex8) of the disk, the refractive index (n) of the disk substrate, the thickness (d) of the disk substrate, and the NA of the objective lens, as expressed by formula (2):                               W          31                =                                            -                              d                2                                      ·                                                                                n                    2                                    ⁡                                      (                                                                  n                        2                                            -                      1                                        )                                                  ⁢                sin                ⁢                                  xe2x80x83                                ⁢                θ                ⁢                                  xe2x80x83                                ⁢                cos                ⁢                                  xe2x80x83                                ⁢                θ                                                              (                                                            n                      2                                        -                                                                  sin                        2                                            ⁢                      θ                                                        )                                                  5                  /                  2                                                              ⁢                      NA            3                                              (        2        )            
To ensure an allowable coma aberration W31 with respect to the tilt of disk for high density recording, there is a tendency to reduce the thickness (d) of the disk substrate. For example, CDs have a thickness of 1.2 mm, and DVDs have a thickness of 0.6 mm. Also, it is likely that the thickness of high-definition DVDs (HD-DVDs), which are recently being developed, will be 0.6 mm or less. In using such thin optical disks for high density recording, the first consideration is the compatibility with existing disks including CDs and DVDs. However, for DVD-recordable (DVD-R) and multi-layered DVDs, it is necessary to use a red light source because of the low reflectivity of these disks with respect to short wavelength light. Therefore, a red light source is used for these disks. With regard to the objective lens, by applying a technique capable of correcting both chromatic aberration caused by the different wavelengths of the light sources, and spherical aberration caused by the difference in thickness of the disks, it is possible to use a common objective lens for both existing disks and HD-DVDs.
For an optical pickup for 0.6 mm-thick DVDs, which uses a red light source and an objective lens that has an NA of 0.6, in order to be compatible with 1.2 mm-thick CDs, an additional light source that has a wavelength of 650 nm and an objective lens are employed in the optical pickup using various techniques. These techniques include an annular shielding technique for blocking light passing through the intermediate area between a far axis and near axis areas, a method of controlling the NA of the objective lens by using liquid crystal (LC) shutters, and a method of splitting light using a hologram optical member to form individual focuses onto two disks that have different thicknesses. However, for compact disk recordables (CD-Rs), reflectivity with respect to red light sharply drops, and thus a light source that has a wavelength of 780 nm is necessary. For this reason, use of a DVD indefinite/CD definite optical system providing compatibility between light beams of 780 nm and 650 nm, or use of an annular objective lens that has an annular focus region between the near axis and far axis regions has been suggested. In particular, for a CD definite optical system, the NA of the objective lens is limited and divergent rays are incident on the objective lens, thereby correcting an aberration caused by thickness variations of disks and the objective lens.
As previously described, an optical pickup using a short wavelength light source is required for higher density information writing and reading than DVD systems are capable of. As an example, for an optical pickup for HD-DVDs, a laser that has a wavelength shorter than 650 nm (as used for DVDs) is required as a light source. In addition, refractivity of optical material for an optical disk varies sharply at wavelengths shorter than 650 nm, thereby causing excessive aberration. Thus, there is a need for an optical system capable of both effectively reducing the chromatic aberration, and being compatible with existing DVDs.
For a DVD-R, the reflectivity with respect to light sources other than a red light source decreases. Thus, for the compatibility with DVD-Rs, a light source that has a wavelength of 650 nm must be used. However, the aberration problem cannot be eliminated from a 400 nm-objective lens just by controlling the degree of divergence of the incident light, which is emitted from the 650 nm-light source and is incident on the objective lens. Thus, a major concern in searching HD-DVDs is an effective chromatic aberration correction technique.
According to a conventional method for correcting aberration, as shown in FIG. 1, an aberration correcting lens 3 is interposed between an optical disk 1 and an objective lens 3. Alternatively, as shown in FIG. 2, a wavelength selecting member 4, which defines the NA of the objective lens 2 depending on the wavelength of light, is located between the objective lens 2 and the light source (not shown). For the optical pickup shown in FIG. 1, the spherical aberration is corrected by adjusting the distance between the two lenses according to the thickness variation of the optical disk 1. For the optical pickup shown in FIG. 2, the NA of the objective lens 2 with respect to light that has a longer wavelength is limited by the wavelength selecting member 4, and its optical elements are arranged such that the light from light sources is divergently incident on the objective lens 2, thereby correcting spherical aberration due to the thickness variation of optical disks.
However, the conventional optical pickup shown in FIG. 1 needs to precisely control the distance between the objective lens 2 and the aberration correcting lens 3. Thus, an additional actuator is required to accurately control the distance between the two lenses 2 and 3, and between lens 3 and the disk. This solution complicates the manufacture of the optical pickup, and thus increases the cost. In addition, for the conventional optical pickup shown in FIG. 2, because a divergent light is used, axial distortion of the objective lens with respect to the disks causes distortion of a tracking error.
To solve the above problems, it is an objective of the present invention to provide an optical pickup that uses laser beams each that have different wavelengths, and is compatible with different types of optical recording media.
It is another objective of the present invention to provide an optical pickup that is capable of being assembled easily with a simple structure, and is compatible with different types of optical recording media.
It is still another objective of the present invention to provide an optical pickup that is capable of being manufactured at low cost, and is compatible with different types of optical recording media.
Additional objects and advantages of the invention will be set forth in part in the description which follows and, in part, will be obvious from the description, or may be learned by practice of the invention.
The objectives of the present invention are achieved by an optical pickup comprising: first and second light sources to generate laser beams, which have different wavelengths, corresponding to first and second media that have different thicknesses, respectively; an objective lens, which has diverging and focusing portions, to focus the laser beams from the first and second light sources onto the first and second media, respectively; at least one photodetector to receive the laser beams that are emitted from the first and second light sources and reflected, respectively, by the first and second media; and a collimating lens arranged on the optical path between the objective lens, and the first and second light sources.
Preferably, assuming that the front focal lengths of the diverging and condensing portions of the objective lens are f1 and f2, respectively, and the Abbe""s numbers on the d-line of the optical materials for the diverging and condensing portions of the objective lens are v1 and v2, respectively, the objective lens satisfies the relationship 0 less than 1/(f1xc2x7v1)+1/(f2xc2x7v2) less than 0.005.
Preferably, assuming that the focal length of the entire objective lens is f and the focal length of the diverging portion is fn, the objective lens satisfies the relationship xe2x88x920.8 less than f/fn less than 0.
Preferably, one surface of the objective lens is an aspheric plane, and the photodetector receives the laser beams originating from both the first and second light sources.
Preferably, the first light source generates a blue laser beam, and more preferably, has a wavelength of 405 nm, and the second light source generates a red laser beam, and more preferably, has a wavelength of 650 nm. Preferably, the substrate of the first medium has a thickness less than 0.6 mm and the substrate of the second medium has a thickness of 0.6 mm.